ART: Use references in assembler tests

Move parameters to const references.

Bug: 32619234
Test: m
Change-Id: Ib68bdc313b91fee1e9e4e1e794eeca630837b005
diff --git a/compiler/utils/mips/assembler_mips32r6_test.cc b/compiler/utils/mips/assembler_mips32r6_test.cc
index 3ef2f94..a52f519 100644
--- a/compiler/utils/mips/assembler_mips32r6_test.cc
+++ b/compiler/utils/mips/assembler_mips32r6_test.cc
@@ -219,7 +219,7 @@
   void BranchCondTwoRegsHelper(void (mips::MipsAssembler::*f)(mips::Register,
                                                               mips::Register,
                                                               mips::MipsLabel*),
-                               std::string instr_name) {
+                               const std::string& instr_name) {
     mips::MipsLabel label;
     (Base::GetAssembler()->*f)(mips::A0, mips::A1, &label);
     constexpr size_t kAdduCount1 = 63;